British Westcountry slang for an ineffectual male. The epithet carries with it implications that the recipient is also impotent, effeminate and incompetent. The origin is unclear, perhaps deriving from the expression 'not worth Molly's piss' meaning to be of no account or worthless, although who Molly was we'll never know.

Malcolm's got all emotional again 'cos he can't get the computer to work, says it's something technical.
Did he plug it in?
No, but he threatened to raise a grievance against me 'cos I laughed at him!
What a Molly Piss!